The broad, long-term objective of this proposal is to examine the role of bone marrow (BM)-derived circulating endothelial precursor cells (CEPs) in the regulation of post-natal angiogenic physiological processes. In particular, we plan to define the mechanism(s) whereby expression of vascular endothelial growth factor receptors, VEGFR2 (Flk-1, KDR) and VEGFR-1 (Flt-1) regulate survival, proliferation, mobilization and recruitment of CEPs during physiological processes such as wound healing or vascular trauma. Despite recent identification of angiogenic factors that regulate embryonic development, the role of angiogenic factors and effector cells that modulate post-natal angiogenesis is not well known. We have identified a population of circulating AC133+VEGFR2+ CEPs that have the capacity to be recruited from BM to the injured vascular tissue, accelerating the angiogenic processes. We have shown that signaling through VEGFR2 is essential for the proliferation and differentiation of CEPs. In addition, VEGF induces mobilization angiogenesis. We have also shown that the angiogenic defect (tumor growth, Matrigel vascularization) in Id1 and Id3 knock out mice (Id1+/-Id3-/-) can be reversed by BM transplantation, suggesting that BM derived CEPs and/or HCs, may play an essential role in the promotion of post-natal angiogenesis. In addition, we speculate that the primary angiogenic defect in (Id1+/-Id3-/-) mice is due to dysregulated VEGF/VEGFR2 and possibly VEGF/VEGFR1 signaling, resulting in a failure of mobilization and proper recruitment of the CEPs and/or HCs to the sites of active neo-angiogenesis. Based on these studies, we hypothesize that VEGF42 and VEGFR1 signaling are essential for the survival, proliferation and mobilization of marrow-derived VEGF2+ CEPs and VEGFR1+ hematopoietic cells (HCs) to the angiogenic vascular bed. Recruitment of CEPs and VEGF-responsive HCs facilitate rapid temporal, spatial and regional expression pattern of VEGFR2 and VEGFFR1 on pre-existing endothelial and recruited BM derived CEPs and HCs during post-natal angiogenesis, 2) Define the role of VEGFR2 and VEGFR1 signaling in the regulation of mobilization, survival and recruitment of CEPs, 3) Assess the physiological significance and contribution of BM-derived VEGFR2+ CEPs and VEGFR1+ HCs to post-natal angiogenesis. These experiments will lead to understanding the mechanism(s) whereby CEPs contribute to the post-natal physiological angiogenic processes. Identifying factors that regulate VEGFR2 and VEGFR1 expression will allow for optimal mobilization, and facilitate recovery of CEPs. These studies will set forth the stage for ex vivo expansion of this rare population of cells that may ultimately be used clinically to accelerate vascular healing.